This invention relates to the field of phase locked loops and, more particularly, to means of providing fast "pull-in" even if the locking signal and VCO reference frequency differ by a frequency substantially higher than the bandwidth of the control filter.
A conventional phase locked loop may have a relatively narrow "lock-in" range, determined largely by the band-width of the filter. Considerations of stability and pull-in time also limit the range of a PLL. There have been many modifications made to PLL circuits over the years since they were first developed. One of these was the "quadricorrelator" which was developed for use with the NTSC color television signals to provide automatic color synchronization. The quadricorrelator included a beat detector and means for changing the bandwidth of the filter when the oscillator was not locked to the frequency of the incoming signal. In another improvement circuit, a plurality of NAND gates was utilized to detect the trailing edges of the two input signals and provide a DC signal to the VCO. While partially successful, this circuit provides a small "dead spot" in the close-in range, and thus will not provide the phase accuracy necessary for cases where the phase relationship between the VCO and the locking signal must be accurately maintained.